White Island is continuing to produce “vigorous bursts” of mud, rock steam and gas up to 100m high and larger explosions including molten sulphur are possible, say experts.

The seismic activity, which has lasted several days, poses a greater than usual risk to visitors, said GNS Science.

Scientists monitoring the island volcano in the Bay of Plenty say the crater lake is drying out and the frequent bursts of mud, steam and gas were still vigorous.

Past monitoring of the island showed new eruptions often develop from this type of activity and small muddy lakes sometimes form in craters “as the unrest develops geysering and explosions occur through these lakes, becoming more vigorous with time”.

White Island
This week downgraded from alert level two to one (out of five). A recently-established lava dome has stopped growing, but scientists say a column of magma not far beneath still poses a threat.

Mt Ruapehu
Remains at alert level one with an exclusion zone around the summit. A suspected blockage below the crater lake may be causing a gas build up that could result in sudden eruption.

Mt Tongariro
Remains at alert level one with part of the Tongariro Alpine Crossing closed. Emitting large amounts of gas and sulphur dioxide and scientists still expect a repeat of the last eruption in November.

Just like Mt Tongariro’s surprise bang on Wednesday afternoon, what happened a few kilometres away at 8.20pm on September 25, 2007, came suddenly and violently.

Shortly before airline pilots noticed a black plume rising above Mt Ruapehu, a volcanic blast threw ash, rocks and water across the summit area, sending two muddy torrents down the skifields.

Inside a hut on the edge of the crater lake, William Pike and James Christie heard a “massive boom” before the building’s door was blown from its hinges and mud and rock poured inside. Mr Pike’s crushed leg later had to be amputated.

The warning signs Mt Ruapehu gave in the days before that explosive moment are being seen again now – worrying scientists that the mountain could be about to produce a similar-sized eruption.

GNS volcanologist Michael Rosenberg said Mt Ruapehu has been showing two forms of unrest, which are considered unrelated.

There have been 45 earthquakes about 5km beneath the mountain since early August, but 35 of those have come in the past month.

GNS Science duty volcanologist Steven Sherburn said changes in measurements at Ruapehu over the last few weeks indicate eruptions are “more likely over the next weeks to months”.

“We are monitoring Ruapehu closely, but it often does not give any immediate warning that it is going to erupt,” Dr Sherburn said.

“We think that the temperature a few hundred metres beneath Crater Lake is about 800 degrees Celsius, but the lake itself is only about 20 degrees Celsius. This suggests the vent is partly blocked which may be leading to a pressure build-up beneath Crater Lake.

“A sudden release of the pressure may lead to an eruption.”

As a result, the Aviation Colour Code has increased from Green to Yellow. However, the Volcanic Alert Level remains at 1. Code Yellow indicates a volcano is experiencing signs of elevated unrest above known background levels.

GNS said small earthquakes have been occurring about 5km beneath the summit area of Ruapehu since late-October, but these may not be directly related to the high temperatures beneath Crater Lake as the earthquakes are much deeper.

A build-up of pressure beneath Crater Lake is thought to have caused the 2007 eruption and a smaller eruption in 2006.